Many of the same concerns effecting fabrication of LIGA molds described in co-pending, commonly owned U.S. patent application Ser. No. 11/112,927 originally filed 22 Apr., 2005 entitled “Aluminum Resist Substrate for Microfabrication by X-ray Lithography and Electroforming” (the entire disclosure of which is herein incorporated by reference) also effect fabrication of the X-ray masks described herein. Both are generally prepared by microfabrication based on X-ray lithography wherein a thick X-ray resist, usually poly(methyl methacrylate) (“PMMA”), is bonded or cast onto a conductive substrate and exposed to X-ray radiation through a patterned mask. The resist is subsequently developed, yielding a patterned mold attached to the substrate that is later filled with a metal deposit. An abbreviated process is illustrated in FIG. 1.
In particular, the substrate carrying the X-ray resist of both mold and mask provides several critical functions including holding resist features in their correct relative positions following development and throughout the plating process. Furthermore, the substrate can dramatically affect the integrity of the resist during X-ray. As illustrated in FIG. 2, primary X-rays from the synchrotron source may pass through the mask membrane and the mask absorber when the photon energy is large. These high-energy photons likely pass through the shadow-region resist, but may be absorbed in the substrate below due to the relatively high X-ray cross-sections of most substrate materials. This absorption produces high-energy photoelectrons and Auger electrons that can leave the substrate, enter the resist, and impart a large secondary radiation dose (>100 J/cm3) in a thin layer of the resist directly adjacent to the substrate surface. The thickness of this layer is typically a few micrometers.
Such interface doses may lead to loss of adhesion through dissolution of the interface during development or through direct radiation-induced degradation of bond is strength. While increasing the mask absorber thickness reduces interface doses, it also degrades feature tolerances and limits producible feature sizes due to inherent limitations of the mask-making process.
X-rays absorbed in “bright” regions of the substrate (i.e., those regions which are unobstructed by an absorbing mask structure and thus “illuminated” by the X-ray source) also affect the LIGA process. Here photons pass through the mask membrane and the resist, where they are absorbed into the substrate holding the resist. Depending on the X-ray cross-section of the resist substrate this absorption can produce high-energy electrons and a high interface resist dose as is illustrated in FIG. 2. In the bright region, however, interface doses may be extremely large (˜100 kJ/cm3) since the mask absorber does not shade these regions and thus removes none of the primary spectrum. Such extreme doses yield a thin layer of insoluble resist adjacent to the substrate, and this insoluble layer must be removed following development via a post-development etch process that roughens the sidewalls of the resist features and is, therefore, unacceptable for some applications of the process.
X-rays absorbed in bright regions of the substrate also produce fluorescence radiation. In contrast to electrons, fluorescence photons are absorbed over large distances, and the range of this absorption may reach several millimeters. Since fluorescence is emitted isotropically, a portion of this radiation is absorbed in the region of the resist that is shaded by the absorbing structures of the mask. During subsequent development, this extraneous dose can lead to substantial dissolution of feature sidewalls and associated errors between the mask pattern and the developed mold geometry. Fluorescence is also produced in shadowed portions of the substrate, but this is generally unimportant due to the much lower primary doses.